1. Field of the Invention
The present disclosure relates to a technology for an improvement in optical output from a vertical type gallium nitride (GaN)-based light emitting diode using an Ag-based highly reflective p-type electrode, quasi-photonic crystal, and surface roughening.
2. Discussion of the Background
With various advantageous features, such as high energy conversion efficiency, long operational lifespan, high directionality, low voltage operation, elimination of preheating time and complex drive circuit, high resistance to impact and vibration, GaN-based white light emitting diodes enable implementation of high quality lighting systems in various ways, and it is anticipated that the GaN-based white light emitting diodes, i.e. a solid-state lighting source, will replace existing light sources, such as incandescent lamps, fluorescent lamps, and mercury lamps, within 5 years. For the GaN-based light emitting diode to be used as a white light source, so as to function as a substitute for existing mercury or fluorescent lamps, it is necessary to have not only excellent thermal stability but also high optical output at low power consumption.
Current horizontal GaN-based light emitting diodes widely used as a white light source in the art have merits such as relatively low manufacturing costs and a simple manufacturing process, but have a fundamental problem in that they are not suitable for a high output light source which requires high operation voltage and a wide area. Vertical light emitting diodes have been developed to solve the problem of the horizontal light emitting diode while allowing easy application to a large area, high output light emitting diode. This vertical light emitting diode has various advantages over existing horizontal light emitting diodes.
The vertical light emitting diode may achieve very uniform current spreading due to low current spreading resistance, thereby providing high optical output at lower operation voltage. Further, since the vertical light emitting diode allows efficient heat discharge through a metal or semiconductor substrate having good thermal conductivity, the vertical light emitting diode exhibits a longer operational lifespan and allows remarkably improved, high output operation. It seems certain that such a vertical light emitting diode will be widely used as a white light source given the 3-4 fold increase in maximum applicable electric current, as compared to horizontal light emitting diodes. Accordingly, not only leading foreign light emitting diode companies, such as NICHIA CHEMICAL CO., LTD. in Japan, PHILIPS LUMILEDS CO., LTD. in the U.S.A., and OSRAM OPTO SEMICONDUCTORS GMBH in Germany, but also domestic companies, such as SEOUL SEMICONDUCTOR CO., LTD., SAMSUNG ELECTRONICS CO., LTD., LG INNOTEK CO., LTD., have researched commercialization and performance improvement of the vertical GaN-based light emitting diodes. Particularly, some companies including OSRAM OPTO SEMICONDUCTORS GMBH have already provided related products to the market.
In order for such a vertical GaN-based light emitting diode to have improved optical output, a p-type ohmic electrode layer formed between a substrate and a p-type semiconductor layer must be a highly reflective electrode which can achieve total non-absorptive reflection. Accordingly, Au-based p-type electrodes such as Ni/Au, Pd/Au, and Pt/Au, which are widely used in existing horizontal light emitting diodes, cannot be used for the vertical light emitting diodes due to their low reflectivity of 40% or less. Therefore, it is necessary to form an Ag-based p-type electrode exhibiting low contact resistance, good thermal stability and high reflectivity in order to improve performance of the vertical light emitting diode.